An overview of microscale indentation fatigue: Composites, thin films, coatings, and ceramics.

There are many applications from computer hardware and sensors to thin films and coatings in which parts are fabricated in small sizes and low thicknesses. Most of these applications could undergo cyclic loading and unloading conditions during their operation. Therefore, cyclic and fatigue evaluations of these components are an essential topic and should be fully understood. In these cases, due to the dimensional limitations, conventional methods of the fatigue experiments encounter challenges and mostly are not accurate or applicable. Nano- and micro-indentation fatigue tests are considered non- or semi-destructive experiments that have opened a new approach to study the cyclic response of these small-sized specimens and thin films. The objective of the present review paper is to evaluate a convenient, reliable, and non-destructive testing approach in the assessment of fatigue (cyclic) response of materials on a small scale. Along with conventional bulk scale fatigue testing methods (i.e. reverse bending, pull-push, multi-axial bending), the depth-sensing indentation testing technique can be employed to study the cyclic behavior of metallic and non-metallic materials especially when a limited volume of the material is available. In this paper, we tried to cover most of the previous studies performed on indentation fatigue of composites, thin films, coatings, and ceramics along with associated discussions and main findings. We covered the physics behind the indentation and the difference between the indentation and conventional fatigue analyses. Followed by that, microstructural evaluations of some of the studies are provided to give readers more insights into this approach. In most applications, the indentation fatigue technique could be a reliable solution due to its accuracy, simplicity, and nondestructive approach in finding out the fatigue and cyclic behavior of materials having a small size or volume. It is worth noting that the loading mode in the indentation fatigue is completely different than the traditional (bulk-scale) fatigue as the tensile segment of the load cycle is not produced in the indentation fatigue (it is a compression-compression loading cycle). Therefore, the controlling mechanisms of failure between small-scale fatigue and bulk-scale fatigue may not be the same.

Electrophoresis of active Janus particles.

We theoretically consider the dynamics of a self-propelled active Janus motor moving in an external electric field. The external field can manipulate the route of a Janus particle and force it to move toward the desired targets. To investigate the trajectory of this active motor, we use a perturbative scheme. At the leading orders of surface activity of the Janus particle and also the external field, the orientational dynamics of the Janus particles behave like a mathematical pendulum with an angular velocity that is sensitive to both the electric field and surface activity of the motor.

Conductance and Fano factor in normal/ferromagnetic/normal bilayer graphene junction.

We theoretically investigate the transport properties of bilayer graphene junctions, where the ferromagnetic strips are attached to the middle region of the graphene sheet. In these junctions, we can control the band gap and the band structure of the bilayer graphene by using the bias voltage between the layers and the exchange field induced on the layers. The conductance and Fano factor (F ) are calculated by the Landauer­Büttiker formula. It is found that when the voltage between the layers or the exchange field are tuned, the pseudodiffusive (F = 1/3) transport turns into tunneling (F = 1) or ballistic transport (F = 0). By tuning the potential difference between the layers, one can control the spin polarization of the current.

Changes in dentofacial structures in untreated Class II division 1 and normal subjects: a longitudinal study.

The purpose of this study was to compare longitudinally the changes that occur in dentofacial structures from the deciduous to the permanent dentitions in untreated Class II division 1 and normal individuals. Complete records were assembled for 65 subjects at three stages of development: at the completion of the deciduous dentition, after the first permanent molars had erupted completely, and after the permanent dentition had erupted completely (third molars excluded). On a cross-sectional basis, only mandibular length (Ar-Pog) differed significantly in the two groups, and then only during the earlier stages of development; by the later stage, the difference was not significant, indicating that some "catch up" growth may occur in Class II individuals. Longitudinal comparisons of the curve profiles, i.e., growth trends between Class II division 1 and normal subjects, indicated that there were no significant differences between the two groups except in upper lip protrusion. Comparisons of the total change from the deciduous to the permanent dentition indicated the presence of a number of significant differences between Class II division 1 and normal subjects, including larger magnitude of maxillary and mandibular lengths in the normal group and greater skeletal and soft tissue convexities in the Class II group.

Longitudinal comparisons of dental arch changes in normal and untreated Class II, Division 1 subjects and their clinical implications.

The purpose of this study was to determine on a longitudinal basis whether the growth trends in maxillary and mandibular dental arch widths and lengths in persons with Class II, Division 1 malocclusions were different from those of normal subjects. Dental casts were available on a total of 37 subjects with Class II, Division 1 malocclusions (15 males and 22 females) and 55 normal subjects (28 males and 27 females). For each subject, three sets of casts were evaluated at the following developmental stages: (1) deciduous dentition (mean age = 5.0 years); (2) mixed dentition (mean age = 8.0 years); and (3) permanent dentition (mean age = 12.5 years). Eighteen dental arch lengths and widths parameters were measured on both the maxillary and the mandibular arches. Interexaminer and intraexaminer reliability were predetermined at 0.5 mm. The Analysis of Variance general linear models procedure was used to compare the various dental arch parameters on both a cross-sectional and a longitudinal basis. Curve parallelism and magnitude were evaluated. The results of the analysis of variance comparing the growth curves of the various dental arch parameters in the normal subjects and untreated subjects with Class II, Division 1 malocclusions indicated the absence of significant differences between the groups, i.e., the curves were parallel (p < 0.01). This finding indicates that the growth trends in the various dental arch parameters evaluated were similar in the normal and Class II, Division 1 malocclusion groups in both male and female subjects. In addition, the differences between the measurements of maxillary and mandibular intermolar arch widths were greater in the normal male subjects than in the male subjects with Class II, Division 1 malocclusions. Female subjects had similar trends but the differences were not statistically significant. In conclusion, the present findings indicate that the clinician should assume that the changes in arch lengths and widths in both the subjects with Class II, Division 1 malocclusions and the normal subjects follow the same general patterns. On the other hand, the relative constriction of the maxillary arch as related to the mandibular arch, In Class II malocclusions is expressed from the earlier stages of dental arch development. These trends continue in the mixed and early permanent dentitions and do not self correct without treatment. Therefore when such trends are diagnosed, early correction of transverse discrepancy should be attempted together with that of the anteroposterior discrepancy.

Comparisons of the dental arch changes in patients with Class II, division 1 malocclusions: extraction vs nonextraction treatments.

Treatment and posttreatment changes in the dental arches of patients with Class II, division 1 malocclusions were evaluated. Half the patients (N = 46) were treated with a nonextraction approach; treatment for the other half (N = 45) included the extraction of four first premolars. The following parameters were measured in the maxillary and mandibular arches: arch width at the incisors, canines, second premolars and first permanent molars; anterior, posterior and total arch lengths; and total and anterior tooth size-arch length discrepancies. These parameters were measured pretreatment, immediately following orthodontic treatment and at least 2 years posttreatment. The findings indicate that the extraction of premolars significantly improves the discrepancy between the tooth size and arch length during treatment. Posttreatment both the extraction and nonextraction groups experienced an increase in tooth size-arch length discrepancies and a reduction in arch length. Extractions did not significantly alter the direction of the overall posttreatment trends for some parameters, e.g., intercanine width and tooth size-arch length discrepancies. On the other hand, the posttreatment trends for other parameters--such as intermolar width--were significantly different between the extraction and nonextraction groups. In general, the trends in the posttreatment changes were similar in males and females as well as in the maxillary and mandibular arches.